1. Field of the Invention
The present invention relates to a circuit for sensing current in a motor controller, an uninterruptable power supply or any system that uses a half bridge circuit and, more specifically, to a circuit for sensing load current by sensing a shunt voltage and transmitting that information to ground level.
2. Description of the Related Art
Resistive shunts are widely used as inexpensive circuit elements for sensing the current passing through a device. The shunt is placed in series with the device for which current is to be sensed. Thus, the voltage drop across the shunt is proportional to current flowing through the shunt and the device.
FIG. 1 shows a typical prior art circuit using a resistive shunt for sensing current through a motor driven by a three phase motor controller. The total current of the three phase currents, i.sub.a, i.sub.b, and i.sub.c, is sensed by sensing the voltage across a common shunt resistor R. This single voltage level is then "decomposed" into the three motor currents of i.sub.a, i.sub.b, and i.sub.c using mathematical techniques in the microprocessor 10 using information such as when each switch in the motor controller is ON.
The disadvantage of the above-described method for sensing load current is that it fails under certain load and speed conditions, and thus is not a robust method.
Another method of sensing load current is to use Hall effect devices to sense the current flow in each phase, as shown in FIG. 2. In this method, each of the Hall effect devices 20, 22 and 24 measures the flux created by the motor current in its respective phase. The disadvantage of using Hall effect devices is that such devices are expensive.
Another scheme that can be adopted to sense load current in a motor controller circuit (shown in FIG. 3 for one of the three phases) is to add a resistor 30 in series between the low side switch and ground. The voltage across resistor 30 is sensed and a sample and hold (S/H) element 32 is used to reconstruct the waveform.
The disadvantages of the scheme shown in FIG. 3 are as follows:
1. the resistor 30 has a parasitic inductance 34 (shown in series with the resistance) which causes a large voltage spike at each turn-on and turn-off instant of the waveform (FIG. 3A). The amplitude of this spike can be as high as 100 times the signal value;
2. the waveform is inherently "choppy;"
3. the voltage spike caused by inductance 34 of resistor 30 can destroy the driver 36 if there is a short circuit on the high side switch; and
4. although the above problems may be minimized using a resistor, R, having a low parasitic inductance, such resistors are very expensive.